Many electronic devices, for example, mobile phones or personal digital assistants (PDAs), normally include an FM function. To simplify a structure of an electronic device, earphone pins of the electronic device generally serves as an FM antenna, for example, a grounding line of the earphone forms the FM antenna or left and right channel lines of the earphone cooperatively form the FM antenna.
When a grounding pin of an earphone jack serves as a signal receiving point of the FM antenna, the grounding pin generally includes an inductor for isolating the FM signal and a reference grounding point of an audio signal. The inductor has an impedance, which will lead to a sharply deteriorating of audio distortion. When a left channel pin and a right channel pin of the earphone jack cooperatively serve as the signal receiving point of the FM antenna, once the earphone is received in the earphone jack, the left and right channel lines of the earphone maybe surrounded by the ground, which will effect a receiving of the FM antenna.